Remove default string value for 'syslog_aggregator' property.

Unfortunately, it doesn't appear like I can reopen #103 so I'm submitting this simple PR.

The default value for the 'syslog_aggregator' property in the ccng spec is breaking many of the templates in the ccng job. It was suggested that 920bb53 would fix this problem but this "safer" check will still fail because the Ruby String class doesn't have the method 'address' (the default value provided in the spec is a string).

If you read the description for this property, having a default doesn't make any sense. "If not defined, cc will not log to syslog" this property can't be undefined because it has a default value.